As information technology advances, more and more information is handled by computer systems, which has led to a rapid increase in capacity of disks for storing the information. As the capacity of disks has increased, the problem of data loss upon disk breakdowns has become more serious. RAID is a known technology for preventing data loss caused by disk errors. There are several RAID levels. The most widely used levels are RAID1, RAID4, and RAID5, and these RAID levels can overcome a breakdown in up to one disk. In contrast, RAID6 uses two parities for each stripe and can overcome a breakdown in up to two disks. Therefore, breakdown immunity with a plurality of disks can be guaranteed by disk management under RAID6, which can overcome a breakdown in up to two disks, instead of RAID1, RAID4, and RAID5, which can overcome a breakdown in up to one disk.
Therefore, if a RAID group managed in a RAID1, RAID4, or RAID5 level can be easily changed into a RAID group under a RAID6 level, breakdown immunity with a plurality of disks can be easily improved. There has been a conventional technology for changing a RAID level, e.g., a technology for changing a RAID level by adding a new disk (see Japanese Laid-Open Patent Publication No. 2006-120120), and a technology for changing a RAID level by transferring data to a newly prepared RAID group (see Japanese Laid-Open Patent Publication No. 2006-318017).
With the technology disclosed in Japanese Laid-Open Patent Publication No. 2006-120120, a disk has to be newly added and, with the technology disclose in Japanese Laid-Open Patent Publication No. 2006-318017, an area for storing data for a post-change RAID group has to be prepared in a disk. Therefore, these conventional technologies are not suitable and cannot change a RAID group under the environment with a limited number of disks, e.g., when users cannot afford a disk, or when there is not enough space for installing a disk in an apparatus.